The present invention relates in general to systems for sampling analog waveforms and more particularly to a method and apparatus for controlling the timing of sampling and data storage by such sampling systems.
Sampling oscilloscopes were developed more than twenty years ago to respond to small, fast-changing signals to which conventional oscilloscopes could not respond due to limited bandwidth or risetime characteristics. Sampling is a now well-known technique wherein a signal path is gated for an extremely short period of time to pass the substantially instantaneous amplitude value (voltage sample) of an electrical signal during that period. Each voltage sample taken in this manner is processed by electronic circuits and displayed as a dot on a cathode ray tube screen at an appropriate time and amplitude position. Since a large number of samples are required to reconstruct a waveform, sampling is most practical when the electrical signal is repetitive in nature since in most cases it is impossible to acquire all of the needed samples during a single event or single cycle of the signal. Indeed, one of the advantages of sampling is that at least one sample can be acquired from each of a large number of cycles, and a representative waveform may be reconstructed and displayed therefrom.
Sampling modes are typified in accordance with the timing method used. Sequential sampling is a mode in which a waveform display is comprised of an orderly series of equally spaced dots. Random sampling is a mode in which successive dots may occur at what appear to be random horizontal positions because the sampling timing and signal triggering are unrelated, although it must be pointed out that with random sampling the reconstructed waveform is defined because the sampling intervals are measured and the dots are inserted into the display at substantially correct time positions.
Random sampling systems of the prior art have been adapted for sampling a high frequency, repetitive waveform at random points along several repetitive sections of the waveform. The resulting waveform display is then formed by ordering and graphically displaying sample data according to the relative sample time of each sample with respect to a triggering event, such as a zero crossing, occurring at the same point within each waveform section. The data acquired by this "equivalent time" method of sampling characterizes the waveform with a resolution equivalent to that which would be obtained if only a single section of the waveform were sampled at a much higher sampling rate. However since the sample timing is random, more samples must be taken to obtain a minimum resolution than would be required if the sample timing were periodic.
Sequential sampling systems of the prior art do sample waveforms periodically but are not adapted for equivalent time sampling because sample timing with respect to a triggering event cannot be precisely controlled. In order for sequential sampling to be used for equivalent time sampling the sampling times for successive waveform sections would have to be progressively skewed, with respect to a repetitive triggering event occurring in each waveform section.